A passenger innocently plugged in his phone to charge it and hadn't realised that it was overheating.

The phone began to melt, emitting smoke and activating the fire alarm in a nearby toilet.

The American Airlines flight, from Milan to New York, was about 100km south-west of Ireland when the alarm was raised.

At around 12.15pm, the captain contacted air traffic controllers to seek permission to divert to Shannon. Cabin crew who went to investigate the incident were unable to locate the source of the smoke.

Meanwhile, the passenger who was charging his phone was apparently unaware that it was the cause of the smoke and emergency.

The Boeing 767-300, which was carrying 167 passengers and crew, was cleared to land while Shannon authorities put the airport's emergency plan into operation.

Fire and rescue services were scrambled and crash tenders took up positions. Units of the fire service at Shannon town were also sent to the airport while crews from Ennis were sent to a holding point at Dromoland.

Ambulances and medical teams from Limerick and Ennis were also mobilised.

Heart

When the jet touched down at 12.48pm, fire personnel boarded it. Engineers also carried out an inspection and discovered the mobile phone had caused the smoke.

Authorities were satisfied it was a genuine error by the passenger and he was allowed remain on board while the flight was cleared to leave after four hours at Shannon.

Meanwhile, a holiday jet had to divert to Dublin Airport yesterday after a passenger suffered a suspected heart attack.

EasyJet flight EZ-757 had departed Belfast at 7.34am for Malaga and was about 100km off Wexford when the crew reported that a passenger was displaying symptoms of a heart attack.

They contacted air controllers to request permission to land at Dublin. The airbus landed about 30 minutes later. Paramedics boarded the aircraft and the man was treated before being taken to hospital.

The plane continued its journey, reaching Malaga about four hours late.